A medical device, designed to be subcutaneously implanted, may include e.g. electronic circuits and/or some other electric device and hence it needs, like any other similar device, electric power for its operation. Examples of such medical devices include e.g. electrical and mechanical stimulators, motors, pumps, valves and constriction devices that can support, stimulate or control various body functions of the person in which the medical device is implanted.
Electric power to a medical device implanted in a person's body can as conventional be supplied from an implanted electrical energy storage such as one or more electrochemical cells. Electric power to devices located inside a person's body can also be supplied from outside the body using wireless energy transfer, for example supplying electric power using electrical induction. In wireless energy transfer any needed amount of electrical power can be supplied intermittently or continuously, without requiring repeated surgical operations. The wirelessly supplied power can be used to directly operate an implanted medical device or to charge an implanted electrical energy storage.
A device suited to be implanted can also require electrical signals in order to generally control its operation and its functions such as, in a simple case, to start or stop the operation of the device. For more complicated devices or devices having more complicated functions, the device could also be required to provide feedback, i.e. signals representing the current state of the device or its function or e.g. some value sensed by the implanted device. Such signals can be exchanged with an external device, e.g. a controller. Then, the signals can also be wirelessly transferred.
Wireless supply of power and wireless exchange of signals have for a medical implant the obvious advantage that no electrical line or cable extending through the skin of the person having the implant is required.
Wireless communication of signals of course also requires electrical power. This is of special importance considering implanted devices and the communication should be designed to require as small electrical power and energy as possible from the implants.
Thus, an external energy source can wirelessly transfer energy to an implanted internal power receiver that is located inside a patient's body and is connected to a medical device for supplying power thereto. So-called TET (Transcutaneous Energy Transfer) devices are known in the art that can wirelessly transfer energy in this manner.
A TET device typically comprises an external energy source including a primary coil adapted to wirelessly transfer any amount of electrical energy by electric induction, by inducing a voltage in a secondary coil of an internal energy receiver which may be implanted immediately inside a person's skin. The efficiency of the inductive power transfer is higher when the primary coil is positioned closer to the secondary coil and when the primary coil is more aligned with the secondary coil, i.e. when a symmetry axis of the primary coil is made more parallel to that of the secondary coil.
Typically, the amount of energy required to operate an implanted medical device may vary over time depending on the operational characteristics of the device. For example, the device may be designed to be switched on and off at certain intervals, or otherwise change its behaviour, in order to provide a suitable electrical or mechanical stimulation, or the like. Such operational variations will naturally result in corresponding variations with respect to the amount of required energy.
Furthermore, the position of the external energy source in relation to the implanted internal energy receiver affects the efficiency of the wireless energy transfer, which as indicated above highly depends on the distance and relative angle between the source and the receiver. For example, when primary and secondary coils are used, changes in coil spacing result in a corresponding variation of the induced voltage. During operation of the medical device, the patient's movements will typically change the relative spacing of the external source and the internal receiver arbitrarily such that the transfer efficiency will have a corresponding large variation.
If the efficiency of the wireless energy transfer becomes low, the amount of energy supplied to the medical device may be insufficient for operating the device properly, so that its action must be momentarily stopped. It may of course interfere with the intended medical effect of the medical device.
On the other hand, the energy supplied to the medical device may also increase drastically, if the relative positions of the external source and the internal receiver change in a way that unintentionally increases the efficiency of the wireless energy transfer. This situation can cause severe problems since the implant cannot “consume” the suddenly very high amount of supplied energy. Unused excessive energy must be absorbed in some way, resulting in the generation of heat, which is highly undesirable. Hence, if excessive energy is transferred from the external energy source to the internal energy receiver, the temperature of the implant will increase, which may damage the surrounding body tissues or otherwise have a negative effect on the body functions. It is generally considered that the temperature in the body should not increase more than three degrees to avoid such problems.
It is thus highly desirable to always supply the correct or appropriate amount of energy to an implanted medical device, in order to ensure a proper operation and/or to avoid an increase of temperature. Various methods are known for controlling the amount of transferred energy in response to different conditions in the receiving implanted device. However, the presently available solutions for controlling the wireless transfer of energy to implanted medical devices are lacking in precision in this respect.
For example, U.S. Pat. No. 5,995,874 discloses a TET system in which the amount of energy transferred from a primary coil is controlled in response to an indication of measured characteristics of a secondary coil, such as load current and voltage. The transferred energy can be controlled by varying the current and voltage in the primary coil, the frequency of the alternating current or the coil dimensions. In particular, a change is effected in the saturation point of the magnetic field between the coils, in order to adjust the power transfer efficiency. However, it is not likely that this solution will work well in practice, since a saturation point in the human tissue would not occur, given the magnetic field levels that can be possibly used. Moreover, if the energy transfer rate must be increased considerably, e.g. to compensate for losses due to variations in alignment and/or spacing between the coils, the relatively high electric and magnetic field may be damaging or unhealthy or at least unpleasant to the patient, as is well known.
An efficient system and an efficient method are thus needed for accurately controlling the amount of energy wirelessly transferred to an implanted medical device to ensure a proper operation thereof. Moreover, excessive energy transfer resulting in a raised temperature at the medical device, and/or power surges in the energy transfer should be avoided, in order to avoid tissue damages and other unhealthy or unpleasant consequences for the patient.